The performance of optical transmission links is largely determined by the spectral properties of the transmission fiber, the optical amplifiers, the reconfigurable optical add/drop multiplexers (ROADMs), and various other components included in the links. Spectral characterization of assembled and installed transmission links are extremely important for link performance optimization, troubleshooting, and maintenance. Modern wavelength division multiplexed (WDM) networks offer many challenges for test and measurement equipment, in particular to conducting spectral measurements in active WDM links in which optical amplifiers are designed to carry many WDM channels. Currently, spectral measurements in active WDM links are either measured unloaded, i.e. without WDM channels, or loaded, i.e. with WDM channels.
The unloaded approach is not suitable for measurement techniques, such as a differential phase shift (DPS) method, based on a single scanning channel, i.e. a probe channel. To avoid nonlinear impairments in transmitted signals, the total output power of a typical optical amplifier, which is normally shared among 40 or 80 WDM channels, is constant and usually is limited to about +20 dBm, i.e. about +4 dBm per channel for 40 channel system. Accordingly, attempts to measure spectral properties with a single channel will lead to heavy nonlinear impairments, e.g. stimulated Brillouin scattering (SBS) and self phase modulation (SPM), in the probe channel, which make the probe channel virtually undetectable. Measurement techniques based on broad band sources (BBS), similar to the fixed analyzer method for PMD measurements, have worked well with WDM amplifiers; however, in the absence of WDM channels, amplifier noise might significantly reduce the degree of polarization of the original BBS and jeopardize the accuracy of such measurements. Also, BBS methods are applicable only to spectrally averaged types of measurements.
The loaded approach requires frequency demultiplexing of a probe signal with the WDM channels, which is commonly done with an interleaver at the output of the link. The interleaver is used to block the WDM channels from entering an analyzer and disturbing the test measurements. Unfortunately, spectral intervals, adjacent to the WDM channels on the ITU grid, are simply eliminated from the measurements by the interleaver, whereby important spectral information is lost.
Moreover, both traditional approaches are disruptive for data traffic in the transmission link under test; therefore, they cannot be used for real time correlation between performance and spectral parameters of the link channels, which is important when link parameters are changing, e.g. PMD fluctuations.
An object of the present invention is to overcome the shortcomings of the prior art by providing a testing system for use in an active WDM optical link providing a variety of spectral measurements under normal operating conditions of the fiber link.